A typical fuel system associated with a gas turbine engine includes a fuel pump operable to deliver fuel via a metering valve and a pressure raising valve to a manifold from which the fuel is delivered through nozzles to the engine. Commonly the engine includes a group of nozzles operable to deliver a pilot flow of fuel to the engine and another group of nozzles operable to deliver a mains flow of fuel to the engine. By appropriate division of the metered flow of fuel between these groups of nozzles, enhanced combustion efficiency and a reduction in exhaust emissions can be achieved.
A staging valve is typically used to divide the metered flow of fuel between the groups of nozzles delivering the pilot and mains flows of fuel. For example, US2008/0271456 describes a fuel system incorporating such a valve. Other techniques whereby a metered flow of fuel can be divided between pilot and mains fuel deliveries involve controlling the operation of appropriate valves to determine whether or not fuel is supplied to the nozzles through which the mains flow is delivered at any given time. For example, the valves through which the fuel is supplied to the nozzles may be hydraulically controlled. An arrangement of this type is described in US2009/0320480.